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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
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NASA NTRS2009

Radiation from Relativistic Shocks with Turbulent Magnetic Fields

Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs at shocked region. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the shock. The "jitter" radiation from deflected electrons in turbulent magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants. New recent calculation of spectra with various different Lorentz factors of jets (two electrons) and initial magnetic fields. New spectra based on small simulations will be presented.

Nishkawa, K.↗
NASA NTRS2010

Simulation of Relativistic Shocks and Associated Radiation from Turbulent Magnetic Fields

Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs at shocked regions. Simulations show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields and particle acceleration. These magnetic fields contribute to the electron's transverse deflection behind the shock. The jitter'' radiation from deflected electrons in turbulent magnetic fields has different properties than synchrotron radiation, which is calculated in a uniform magnetic field. This jitter radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants. We will present detailed spectra for conditions relevant of various astrophysical sites of shock formation via the Weibel instability. In particular we will discuss the application to GRBs and SNRs

Nishikawa, K.-I.↗
NASA NTRS2010

Simulation of Relativistic Shocks and Associated Self-Consistent Radiation

Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs at shocked regions. Simulations show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields and particle acceleration. These magnetic fields contribute to the electron's transverse deflection behind the shock. The "jitter" radiation from deflected electrons in turbulent magnetic fields has different properties than synchrotron radiation, which is calculated in a uniform magnetic field. This jitter radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants. We will present detailed spectra for conditions relevant of various astrophysical sites of shock formation via the Weibel instability. In particular we will discuss the application to GRBs and SNRs.

Nishikawa, K.-I.↗
NASA NTRS1973

Transient ion neutralization by electrons.

The nonlinear initial-boundary-value problems describing the lateral neutralization of ion beams for the cases that (1) an auxiliary electric field accelerates the electrons into the ion space, and (2) the electrons are injected into the ion space at a prescribed current density are treated. Analytical solutions are derived which give the position and speed of the neutralization front as a function of time, and the temporal development of the electron density, velocity, and electric fields during the neutralization process.

Wilhelm, H. E.↗
NASA NTRS1982

Anomalous auroral electron distributions due to an artificial ion beam in the ionosphere

Results are reported for the perturbation of the auroral ionosphere by the operation of an ion gun which injected about 100 mA of 25-eV Ar(+) ions at upgoing pitch angles over a discrete auroral arc. The major effects observed were the excitation of intense broadband electric field fluctuations at zero-10 kHz, and the appearance of streaming and isotropic heating in different parts of superthermal electron velocity space. A scenario is explored in which electron runaway or streaming is expected between the trapping speed and the critical velocity for cyclotron interactions with the waves, where the streaming electrons carry the current that would be carried by thermals or energetic electrons in the absence of the waves. A current of about 1.0 microA/sq m is carried by the streaming electrons. The gun-associated electrons were anomalous in the sense that their anisotropy was the opposite of that observed in the natural aurora.

Moore, T. E.↗
NASA NTRS2014

Magnetosheath Filamentary Structures Formed by Ion Acceleration at the Quasi-Parallel Bow Shock

Results from 2.5-D electromagnetic hybrid simulations show the formation of field-aligned, filamentary plasma structures in the magnetosheath. They begin at the quasi-parallel bow shock and extend far into the magnetosheath. These structures exhibit anticorrelated, spatial oscillations in plasma density and ion temperature. Closer to the bow shock, magnetic field variations associated with density and temperature oscillations may also be present. Magnetosheath filamentary structures (MFS) form primarily in the quasi-parallel sheath; however, they may extend to the quasi-perpendicular magnetosheath. They occur over a wide range of solar wind Alfvénic Mach numbers and interplanetary magnetic field directions. At lower Mach numbers with lower levels of magnetosheath turbulence, MFS remain highly coherent over large distances. At higher Mach numbers, magnetosheath turbulence decreases the level of coherence. Magnetosheath filamentary structures result from localized ion acceleration at the quasi-parallel bow shock and the injection of energetic ions into the magnetosheath. The localized nature of ion acceleration is tied to the generation of fast magnetosonic waves at and upstream of the quasi-parallel shock. The increased pressure in flux tubes containing the shock accelerated ions results in the depletion of the thermal plasma in these flux tubes and the enhancement of density in flux tubes void of energetic ions. This results in the observed anticorrelation between ion temperature and plasma density.

magnetosheath↗
NASA NTRS1985

Remote sensing of plasma injection and acceleration phenomena

Dynamics Explorer-1 High Altitude Plasma Instrument data have been used to investigate the injection of magnetosheath plasma into the polar cusp, the injection of auroral ion beams into the magnetosphere, and the acceleration of electrons transverse to the magnetic field direction, and the results are discussed. In the case of polar cusp plasmas, it is found that injection occurs at the high-latitude magnetopause, at geocentric distances near eight earth radii. In the case of auroral ion beams it is determined that ion bands are not produced by equatorial injection from the plasma sheet, but by the upward acceleration of ions from auroral acceleration regions. Finally, conical electron distributions are found to be consistent with transverse acceleration at altitudes of a thousand or more km, within or below the magnetic-field-aligned potential drops of the auroral acceleration regions.

Burch, J. L.↗
NASA NTRS1985

The polar ionosphere as a source of the storm time ring current

The transport of ions from the polar ionosphere to the inner magnetosphere during storm time conditions has been computed using a Monte Carlo diffusion code. The effect of the electrostatic turbulence assumed to be present during the substorm expansion phase was simulated by a process that accelerated the ions stochastically perpendicular to the magnetic field with a diffusion coefficient proportional to the rate of energization of the ions by the induced electric field. This diffusion process was continued as the ions were convected from the plasma sheet boundary layer to the double-spiral injection boundary. Inward of the injection boundary the ions were convected adiabatically. By using as input an O(+) flux of 2.8 x 10 to the 8th per sq cm per s (w greater than 10 eV) and an H(+) flux of 5.5 x 10 to the 8th per sq cm per s (w greater than .63 eV) the computed distribution functions of the ions in the ring current were found to be in good agreement, over a wide range in L (4-8), with measurements made with the ISEE 1 satellite during a storm. This O(+) flux and a large part of the H(+) flux appear to be consistent with the DE-1 and DE-2 satellite measurements of the polar ionospheric outflow during disturbed times.

Cladis, J. B.↗
NASA NTRS1982

The ionosphere as a source for magnetospheric ions

Ion composition measurements within the past several years have shown O(+), He(+), and other ions of terrestrial origin to compose a substantial fraction of the magnetospheric ion population. This review examines (1) observations of topside ionospheric composition, (2) mechanisms for energization and injection of ionospheric ions into the magnetosphere, and (3) observations of ions of ionospheric origin in various regions of the magnetosphere, including the plasmasphere, ring current, magnetotail plasma sheet and lobes, and boundary layer and magnetosheath.

Horwitz, J. L.↗
NASA NTRS1998

Direct Acceleration of Pickup Ions at The Solar Wind Termination Shock: The Production of Anomalous Cosmic Rays

We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed Anomalous Cosmic Ray (ACR) fluxes. A non-linear Monte Carlo technique was employed, which in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross- field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He(+)/H(+) or O(+)/H(+) to within a factor approx. 5. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock is totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much-discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles.

Ellison, Donald C.↗
NASA NTRS1993

Cross-tail current - Resonant orbits

A technique to generate self-consistent 1D current sheets is described. Groups of monoenergetic protons were followed in a modified Harris magnetic field. This sample current sheet is characterized by resonant quasi-adiabatic orbits. The magnetic moment of a quasi-adiabatic ion which is injected from outside a current sheet changes substantially during the orbit but returns to almost its initial value by the time the ion leaves. Several ion and electron groups were combined to produce a plasma sheet in which the charged particles carry the currents needed to generate the magnetic field in which the orbits were traced. An electric field also is required to maintain charge neutrality. Three distinct orbit types, one involving untrapped ions and two composed of trapped ions, were identified. Limitations associated with the use of a 1D model also were investigated; it can provide a good physical picture of an important component of the cross-tail current, but cannot adequately describe any region of the magnetotail in which the principal current sheet is separated from the plasma sheet boundary layer by a nearly isotropic outer position of the central plasma sheet.

Kaufmann, Richard L.↗
NASA NTRS1993

Stormtime transport of ring current and radiation belt ions

A dynamical guiding-center simulation model is used to study the stormtime ion transport which leads to the formation of the ring current and diffusion in the radiation belts. Representative ions guiding-center motion in response to model storm-associated impulses in the convection electric field is traced for a range of ion mu values. The present numerical results are compared with previously formulated limiting idealization of particle transport in order to assess the limits of validity of these approximations. For ions having drift periods that exceed the duration of the main phase of the storm, their inward transport to form the stormtime ring current is appropriately described as direct convective access. For ions having drift periods comparable to the duration of the main phase of the storm, there is a transition between direct convective access and transport that resembles radial diffusion. Lower-energy ring-current ions at L of about 3 are freshly injected there from open adiabatic trajectories, whereas the higher-energy ring-current population consists of a mixture of freshly injected and previously trapped ions.

Chen, Margaret W.↗
NASA NTRS1976

Simultaneous equatorial observations of 1- to 30-Hz waves and pitch angle distributions of ring current ions

Eighteen events of large-amplitude (0.4-6 gammas) waves which may be propagating in the ion cyclotron mode have een observed by Explorer 45. Comparison with simultaneously measured proton distributions has allowed the events to be divided into two categories. The first category consists of waves accompanied by enhanced ion fluxes apparently injected into the plasmasphere with anisotropic pitch-angle distributions. This simultaneity suggests that these waves may be generated by the observed ring-current ions. Waves in the second category were found near or outside the plasmapause and were not correlated with any identifiable changes in the observed proton distribution. The generation mechanism for these waves remains unknown.

Taylor, W. W. L.↗
NASA NTRS1994

Location and characteristics of the reconnection X-line deduced from low-altitude satellite and radar observations

We present an analysis of a cusp ion step observed between two poleward-moving events of enhanced ionospheric electron temperature. From the computed variation of the reconnection rate and the onset times of the associated ionospheric events, the distance between the satellite and the X-line can be estimated, but with a large uncertainty due to that in the determination of the low-energy cut-off of the ion velocity distribution function, f(E). Nevertheless, analysis of the time series f(t) shows the reconnection site to be on the dayside magnetopause, consistent with the pulsating cusp model, and the best estimate of the X-line location is 13 R(E) from the satellite. The ion precipitation is used to reconstruct the field-parallel part of the Cowley-D ion distribution function injected into the open low latitude boundary layer (LLBL) in the vicinity of the X-line. From this the Alfven speed, plasma density, magnetic field, parallel ion temperature, and flow velocity of the magnetosheath near the X-line can be derived.

Lockwood, M.↗
NASA NTRS1987

Neutralization of beam-emitting spacecraft by plasma injection

An impulsive plasma injection has been used to study charge neutralization of the Space Shuttle Orbiter while it was emitting an electron beam into space. This investigation was performed by Space Experiments with Particle Accelerators on Spacelab-1. A plasma consisting of 10 to the 19th argon ion-electron pairs was injected into space for 1 ms while an electron beam was also being emitted into space. The electron beam energy and current were as high as 5 keV and 300 mA. While the orbiter potential was positive before the plasma injection and began to decrease during the plasma injection, it was near zero for 6 to 20 ms after the plasma injection. The recovery time to the initial level of charging varied from 10 to 100 ms. In a laboratory test in a large space chamber using the same flight hardware, the neutralization time was 8-17 ms and the recovery time was 11-20 ms. The long duration of the neutralization effect in space can be explained by a model of diffusion of the cold plasma which is produced near the Orbiter by charge exchange between the neutral argon atoms and the energetic argon ions during plasma injection.

Sasaki, S.↗
NASA NTRS1987

Magnetospheric plasma sputtering of Io's atmosphere

Neutral coronae due to magnetospheric ion sputtering from the exobase are generated, and ion and neutral injection into the plasma torus are calculated, on the assumption of the existence of an atmosphere on Io and a consideration of that atmosphere's various possible characteristics. Through the inclusion of the sputtered atmospheric corona that emerges from an assumed unattenuated incident ion flux, it becomes possible to satisfy the supply rate to the torus with an exobase that is close to the surface; such an exobase would imply that the Io atmosphere is insufficiently robust to support a fully dissociated corona, and that a significant fraction of the incident plasma ions can penetrate to the surface to furnish a sputter source of atmospheric gas.

Mcgrath, M. A.↗
NASA NTRS2018

Magnetosphere Dynamics During the 14 November 2012 Storm Inferred from TWINS, AMPERE, Van Allen Probes, and BATS-R-US-CRCM

During the 14 November 2012 geomagnetic storm, the Van Allen Probes spacecraft observed a number of sharp decreases ('dropouts') in particle fluxes for ions and electrons of different energies. In this paper, we investigate the global magnetosphere dynamics and magnetosphere- ionosphere (M-I) coupling during the dropout events using multipoint measurements by Van Allen Probes, TWINS, and AMPERE together with the output of the two-way coupled global BATS-R-US-CRCM model. We find different behavior for two pairs of dropouts. For one pair, the same pattern was repeated: (1) weak nightside Region 1 and 2 Birkeland currents before and during the dropout; (2) intensification of Region 2 currents after the dropout; and (3) a particle injection detected by TWINS after the dropout. The model predicted similar behavior of Birkeland currents. TWINS low-altitude emissions demonstrated high variability during these intervals, indicating high geomagnetic activity in the near-Earth tail region. For the second pair of dropouts, the structure of both Birkeland currents and ENA emissions was relatively stable. The model also showed quasi-stationary behavior of Birkeland currents and simulated ENA emissions with gradual ring current buildup. We confirm that the first pair of dropouts was caused by large-scale motions of the OCB (open-closed boundary) during substorm activity. We show the new result that this OCB motion was associated with global changes in Birkeland (M-I coupling) currents and strong modulation of low-altitude ion precipitation. The second pair of dropouts is the result of smaller OCB disturbances not related to magnetospheric substorms. The local observations of the first pair of dropouts result from a global magnetospheric reconfiguration, which is manifested by ion injections and enhanced ion precipitation detected by TWINS and changes in the structure of Birkeland currents detected by AMPERE. This study demonstrates that multipoint measurements along with the global model results enable the reconstruction of a more complete system-level picture of the dropout events and provides insight into M-I coupling aspects that have not previously been investigated.

Buzulukova, Natalia↗